Hydraulic vulnerability of Australian forests to drought

Abstract

Climate change threatens native and agricultural systems with increasing drought and elevated temperatures. Forest mortality is on the rise and drought has been implicated in many large-scale events. But what makes some species persist and others perish? Large variation in plants' susceptibility to drought conditions illustrates the complexity of plant drought response. Hydraulic dysfunction in plants has been highlighted as a key mechanism of drought mortality arousing interest in hydraulic traits and thresholds and generating a large dataset of plant species from a wide range of environments, particularly as modellers look for ways to predict and describe forest mortality. Despite the immense interest in the field, several questions remain unresolved. In this thesis I compile a unique dataset of hydraulic traits for Australian forest species using consistent techniques to tackle key knowledge gaps in our understanding of the plant drought response. Specifically, I examine how vulnerable tree species from different forest biomes are to drought and how close they are to experiencing hydraulic dysfunction. I then investigate how the limits of the plant hydraulic system differ within plants by using new imaging methods to dive deeply into some commonly held paradigms and shed light on the true nature of plants' hydraulic system. In this research I illustrate the susceptibility of Australian forests to future climate change. Three components quantify this risk: i) the increase in plant water stress with site aridity, ii) the adaptation of forest species to their unique moisture environment and iii) the convergence of species to narrow safety margins across Australia. Additionally, I discovered that xylem vulnerability is consistent between stems and leaves in Australian forests, contrary to the long-standing vulnerability segmentation hypothesis; this opens up a discussion of method interpretations and leaf-level drought resistance strategies. I then turn my focus underground: using non-destructive imaging, I present evidence that cavitation occurs simultaneously in axial root and stem xylem, casting doubt on a long-held paradigm that roots are more vulnerable than stems. This research highlights within-site variability in drought response strategies, suggesting follow-up work on site-level drought strategies. Additionally, this work stresses the importance of comprehensive mechanistic investigations of water potential drivers and responses, as well as follow-up studies on roots and leaves.

Date of Award	2019
Original language	English